One conventional front fork comprises, as described in Japanese Examined Utility Model Application Publication No. S49-1162, a spring leg which does not have a built-in damper but have a built-in suspension spring formed of a metal spring for suspending a vehicle weight.
The applicant of this invention proposes, as described in Japanese Patent Application Laid-Open (kokai) No. 2012-092945, to use an air spring as the suspension spring incorporated in a spring leg in order to reduce the weight of the front fork and to reduce the vehicle weight.
The spring leg described in Japanese Patent Application Laid-Open (kokai) No. 2012-092945 is configured such that a vehicle body-side tube and an axle-side tube are inserted into each other, a guide cylinder is provided at a central portion of the inside of either one of the vehicle body-side tube or the axle-side tube, a guide rod is provided at a central portion of the inside of the other of the vehicle body-side tube and the axle-side tube, a guide of the guide rod being inserted into the guide cylinder, a piston ring and a seal member are provided on an outer periphery of the guide of the guide rod so as to be slidably in contact with an inner periphery of the guide cylinder, and an inner air spring chamber defined by the guide of the guide rod is formed in the inside of the guide cylinder.
[Patent document 1] Japanese Examined Utility Model Application Publication No. S49-1162
[Patent document 2] Japanese Patent Application Laid-Open (kokai) No. 2012-092945
In the spring leg described in Japanese Patent Application Laid-Open (kokai) No. 2012-092945, there are provided, on the outer periphery of the guide rod, an inward-facing seal member, a piston ring, and an outward-facing seal member sequentially in this order along an axial direction of the guide from the side close to the inner air spring chamber to the side far from it.
The inward-facing seal member has a lip that is slidably in contact with the inner periphery of the guide cylinder and is oriented toward the inner air spring chamber, and the outward-facing seal member has a lip that is slidably in contact with the inner periphery of the guide cylinder and is oriented in the opposite direction from the inner air spring chamber. According to this arrangement, the inward-facing seal member is opened by an air pressure in the inner air spring chamber to prevent leakage of the air pressure and to maintain a spring force of the air spring in the inner air spring chamber. The outward-facing seal member prevents a lubricating oil located in a lower part of the guide cylinder from entering the inner air spring chamber and prevents the air pressure in the inner air spring chamber from being increased to a critical pressure by the increase of the oil entering the chamber.
However, in the spring leg described in the Japanese Patent Application Laid-Open (kokai) No. 2012-092945, the piston ring is located between the inward-facing seal member and the outward-facing seal member on the outer periphery of the guide of the guide rod, and this piston ring produces abrasive dust. The abrasive dust from the piston ring can move both toward and against the inner air spring chamber in the inside of the guide cylinder, depending on orientations of the lips of the inward-facing seal member and the outward-facing seal member, and may scratch the inward-facing seal member and the outward-facing seal member in the course of the movement. If the inward-facing seal member is scratched, a sealability of the inward-facing seal member will be impaired, which will reduce the spring force of the air spring in the inner air spring chamber, possibly leading to an impaired suspension function. If the outward-facing seal member is scratched, a sealability of the outward-facing seal member will be impaired, which will cause the lubricating oil in the lower part of the guide cylinder to enter the inner air spring chamber, and the air pressure in the inner air spring chamber will be increased to the critical pressure by the increase of the oil entering into the chamber.